Single antenna for receipt of signals from multiple communications systems

ABSTRACT

A method and device for using a single antenna for simultaneously receiving radio frequency signals in a first radio frequency band and radio signals in a second radio frequency band different from the first radio frequency band.

[0001] This application claims the benefit of U.S. ProvisionalApplication Serial No. 60/146,880, filed in the names of Brian Lindemannand Tuan Nguyen on Aug. 3, 1999, the complete disclosure of which isincorporated herein by reference.

FIELD OF THE INVENTION

[0002] This invention relates to radio antenna systems and methods,particularly antenna systems and methods for enabling one antenna totransmit a first frequency radio signal and simultaneously receive asecond radio signal of a frequency near the first radio signalfrequency.

BACKGROUND OF THE INVENTION

[0003] Both radio navigation and radio communications equipment areoften employed in the operation of vehicles, such as boats, automobiles,and airplanes. One common type of radio navigation is Global PositioningSystem navigation, commonly known as GPS navigation, which works bycomputing the vehicle's position based on radio signals encoded withephemeris data received from multiple satellites. Satellitetelecommunications systems provide worldwide wireless two-way telephoniccommunications.

[0004] Given the current low cost of both GPS navigation and satellitetelecommunications systems, many marine, automotive, and airbornevehicles carry both systems on-board. Often, the vehicle passengers andcrew find simultaneous operation of both systems desirable.

[0005] GPS navigation data signals are currently broadcast at a carrierfrequency of 1.575 gigahertz (GHz). Two-way satellite telecommunicationssystems operate in the L-band frequency range. The system operatingfrequency is system dependent, with each manufacturer's equipmentoperating in a frequency range assigned by license. For example, onecommercial system operates in the frequency band of 1.616 GHz and 1.625GHz. Although satellite telecommunications systems transmit and receiveat frequencies nearly the same as the GPS operating frequency, thesatellite telecommunications broadcasts and transmissions must notinterfere with GPS navigation, at least as regards aircraft operatingunder FAA regulations.

[0006] Normally, each system operates a dedicated antenna.Traditionally, the requirement of noninterference is satisfied, first byfiltering the satellite telecommunications system transmitter to limitradio frequency (RF) energy in the GPS operating band, and second byattenuating the RF energy produced by the satellite telecommunicationssystem in the GPS operating band by physically separating the GPS andsatellite telecommunications system antennas on the host aircraft. Eachof these responses is unsatisfactory. The traditional filter capable ofsatisfying the requirement is large and heavy, both undesirable traitsin aircraft equipment. Antenna space on most aircraft is minimal,further limitations on the antenna system, such as a minimum distancebetween antennas, exacerbates the problem. Antenna space is similarlylimited in some marine and automotive applications. Thus,notwithstanding the above considerations in response to thenoninterference requirement, employing a single antenna is desirable toboth receive GPS navigation signals and to both receive and transmitsatellite telecommunications system signals.

[0007] Furthermore, a desire toward economy of cost recommends that asingle antenna perform for multiple radio systems. Moreover, a desirableantenna is relatively light weight and low profile to minimize drag andmaximize cosmetic appearances.

[0008] Additionally, a single antenna both transmitting and receivingcommunication signals in a first radio frequency band and simultaneouslyreceiving a second radio signal having a frequency near the first radiofrequency band is desirably versatile enough to be used with a varietyof electronic equipment.

[0009] U.S. Pat. No. 5,170,493 discloses a single antenna used to bothreceive relatively low frequency LORAN-C radio signals broadcast at acarrier frequency of 100 kilohertz (kHz) and to transmit or receiverelatively high frequency VHF radio signals broadcast at carrierfrequencies in the range of 30 megahertz (MHz) up to 300 MHz. U.S. Pat.No. 5,170,493 also describes a number of other systems for employing thesame antenna to both receive relatively low frequency radio signals andto transmit or receive relatively high frequency radio signals byreference to Tanner, et al. U.S. Pat. No. 4,268,805; Elliott, U.S. Pat.No. 4,095,229; and Tyrey, U.S. Pat. No. 4,037,177. Powell, et al. U.S.Pat. No. 3,812,494 discloses a system for receiving and transmittingDoppler frequencies and telemetering frequencies simultaneously over thesame antenna, as distinguished from GPS and satellite telecommunicationssystem frequencies, and employs an impedance matching network in closeproximity with the antenna itself. Duncan, Jr., et al. U.S. Pat. No.3,217,273 discloses a system for coupling several transmitters andreceivers to a single antenna, while avoiding the radiation of spurioussignals resulting from intermodulation, and avoiding swamping thereceivers. In addition, U.S. Pat. No. 3,739,390 discloses a system usingtwo antennas operating within different frequency ranges but sharing asingle coaxial line, the central conductor forming the receiving elementof one antenna and the shield forming the receiving element of the otherantenna, duplexing circuitry connected to the antenna separates thesignals received at the two antennas and applies the separated signalsto the appropriate receiver or transmitter. U.S. Pat. No. 5,574,978discloses an interference cancellation system allowing differenttransmitting and receiving radios to utilize a single antenna.

[0010] Although a number of systems have been developed for employingone antenna to both receive relatively low frequency radio signals andto transmit or receive relatively high frequency radio signals, suchsystems solve only the problem as relates to signals orders of magnitudedifferent in frequency. Each of the disclosed systems fails to eitherconsider or resolve the problem of using one antenna to both transmitand receive signals in a first radio frequency band and simultaneouslyreceive a second radio signal having a frequency very near the firstradio frequency band as presented, for example, by the problem ofcombining the function of receiving GPS navigation signals in a singleantenna with the function of transmitting and receiving satellitetelecommunications system signals where the frequencies are separated byapproximately 41 MHz or only 3%. Furthermore, the systems disclosed inthe prior would not allow sharing of a single antenna with radio systemsoperating in the frequency ranges used by GPS and satellitetelecommunications systems. Nor would the prior art systems allowsharing of a single antenna with two radio systems operating with such arelatively small frequency spread.

SUMMARY OF THE INVENTION

[0011] This invention provides a method of utilizing a single antennafor both a satellite telecommunications system transceiver and a GPSreceiver, the total implementation of which provides less RF loss in thesatellite telecommunications system path, and a smaller filter packagethan traditional implementations.

[0012] The present invention overcomes the limitations of the prior artby providing a method and circuit for using one antenna to both transmitand receive signals in a first radio frequency band and simultaneouslyreceive a second radio signal having a frequency very near the firstradio frequency band.

[0013] According to one aspect of the invention, the circuit receives aradio frequency (RF) signal in a first radio frequency band introducedinto the first port of a directional coupler. The directional couplerdivides the energy in the received signal into two signals andintroduces each of the divided signals into second and third ports ofthe directional coupler. Band pass filters coupled to each of the secondand third ports of the directional coupler reflect any energy in thedivided signals which is in frequency bands other than a seconddifferent radio frequency band. The directional coupler combines theenergy in the reflected signals into a first RF output signal. Thereceived RF signal is either a broadcast signal received at an antennacoupled to the first port of the directional coupler or a transmissionsignal generated by a transmitter that is coupled to the first port ofthe directional coupler. According to one embodiment of the invention,the RF energy reflected by the band pass filters coupled to each of thesecond and third ports of the directional coupler is energy in theL-Band frequency range, the operation band of typical two-way satellitetelecommunications systems. For example, RF energy reflected by the bandpass filters coupled to each of the second and third ports of thedirectional coupler is energy in the frequency band of 1.616 GHz and1.625 GHz, i.e., the operation band of one known two-way satellitetelecommunications system. According to another aspect of the invention,a conventional two-way satellite telecommunications systems transceiveris coupled to the directional coupler to generate and receive signals inthe L-Band frequency range, for example, in the frequency band of 1.616GHz and 1.625 GHz. According to yet another aspect of the invention, aband pass filter is coupled between the output of the directionalcoupler and the transmitter to clean the generated signal of harmonicsin the second frequency range and other spurious emissions.

[0014] According to another aspect of the invention, the band passfilters coupled to each of the second and third ports of the directionalcoupler pass any energy in the divided signals which is in the seconddifferent radio frequency band. A second coupler coupled to the aboveband pass filters combines the passed RF energy into a second RF outputsignal in the second radio frequency band. According to one embodimentof the invention, the RF energy passed by the band pass filters is inthe 1.575 gigahertz (GHz) frequency band. According to one embodiment ofthe invention, the RF energy passed by the band pass filters in the1.575 gigahertz (GHz) frequency band is received and processed by aconventional Global Positioning System receiver. According to variousother aspects of the invention, a low noise amplifier amplifies and aseries of one or more band pass filters coupled between the output ofthe second coupler and the receiver respectively amplify and clean thereceived signal.

[0015] BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The foregoing aspects and many of the attendant advantages ofthis invention will become more readily appreciated as the same becomesbetter understood by reference to the following detailed description,when taken in conjunction with the accompanying drawings, wherein:

[0017] The FIGURE is an illustrative block diagram describing theisolation circuit of the present invention, including a radiotransceiver that transmits and receives signals in a first radiofrequency band simultaneously with a radio receiver receiving a secondradio signal having a frequency very near the first radio frequency bandusing a single antenna.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

[0018] The present invention is, a unique isolation circuit that allowsa first radio system transmitting and receiving signals in a first radiofrequency band to simultaneously share a single antenna with a secondradio system receiving a second radio signal having a frequency verynear the first radio frequency band. An illustrative embodiment of theisolation circuit of the invention is described herein in terms of anisolation circuit allowing a GPS radio navigation signal receiver,commonly known as a GPS receiver, to simultaneously share a singleantenna with a satellite telecommunications system, also known as aSatCom system. The embodiment illustrated is not intended to in any waylimit the scope of the invention. Those of ordinary skill in the artwill recognize that isolation circuit of the invention may be practicedwith other radio systems than those described in the illustration.

[0019] Global positioning system radio navigation, commonly known as GPSnavigation, is well known in the art, as evidenced by several issuedU.S. patents and commercial products embodying the inventions disclosedin such patents. Examples are disclosed in U.S. Pat. No. 5,222,245 andU.S. Pat. No. 5,546,092, the disclosures of which are incorporatedherein by reference. GPS navigation data signals are currently broadcastat a carrier frequency of 1.575 gigahertz (GHz) and received on adedicated antenna coupled to the GPS receiver. Global satellitetelecommunications systems, commonly known as SatCom systems, are alsowell known in the art, as similarly evidenced by several issued U.S.patents and commercial products embodying the inventions disclosed insuch patents. One exemplary SatCom system is disclosed in U.S. Pat. No.5,918,155, the disclosure of which is incorporated herein by reference.The AIRSAT™ 1 multichannel SatCom systems of the IRIDIUM® World AirService and manufactured by AlliedSignal Incorporated are two commercialexamples. Conventional two-way SatCom systems, such as the AIRSAT™ 1multichannel SatCom systems, are known to transmit and receive in theL-Band frequency range assigned by license, specifically in thefrequency band of 1.616 GHz and 1.625 GHz. GPS and SatCom systems eachoperate over an independent dedicated antenna. When present on a singlevehicle, the independent GPS and SatCom antennas are isolated from oneanother, primarily to preclude interference of the GPS signals by theSatCom transmissions.

[0020] In general, the invention makes use of conventional frequencymatched twin-line couplers, small filters and a low noise amplifier toduplex a radio frequency transceiver, such as a SatCom systemtransceiver, and a radio system receiving radio signals broadcast at afrequency very near the transceiver radio frequency band, such as a GPSreceiver, simultaneously on single antenna. Losses occur in anypractical implementation of the invention. While the description belowomits reference to such losses for clarity of explanation, those ofordinary skill in the art will recognize and account for such losses ina practical implementation of the invention.

[0021] The FIGURE is an illustrative block diagram describing theisolation circuit 10 of the present invention. A radio transceiver 12transmits and receives signals in a first radio frequency bandsimultaneously with a radio receiver 14 receiving a second radio signalhaving a frequency very near the first radio frequency band using asingle antenna 16. Radio transceiver 12 is, for example, a conventionaltwo-way SatCom system transmitting and receiving in the L-Band frequencyrange, for example, in the frequency band of 1.616 GHz and 1.625 GHz.Radio receiver 14 is, for example, a conventional GPS receiver receivingGPS navigation data signals broadcast at a nearby carrier frequency of1.575 gigahertz (GHz). Thus, the operating frequencies of transceiver 12and receiver 14 are only separated by approximately 41 MHz or only 3%.Shared antenna 16 has a band width broad enough to simultaneouslyreceive both the GPS navigation signals broadcast at 1.575 GHz andSatCom signals broadcast at 1.616 GHz, and to radiate the 1.625 GHzsignal transmitted by two-way SatCom transceiver 12.

[0022] Antenna 16 is coupled to a four-way directional coupler 18 formedusing conventional wire-line couplers tuned to the desired frequencies.RF energy received at antenna 16 is introduced at port A of coupler 18and coupled into ports C and D. The coupling between ports A and Cresults in a loss of 3 dB with a 0 degree phase shift. The couplingbetween ports A and D results in a loss of 3 dB with a −90 degree phaseshift. Similar couplings exist from B to D and C, from C to A and B, andfrom D to B and A. Port B is similarly coupled into ports D and C, withthe coupling between B and D resulting in a loss of 3 dB with a 0 degreephase shift, and the coupling between ports B and C resulting in a lossof 3 dB with a −90 degree phase shift. Port C is similarly coupled intoports A and B, with the coupling between C and A resulting in a loss of3 dB with a 0 degree phase shift, and the coupling between ports C and Bresulting in a loss of 3 dB with a −90 degree phase shift. Port D issimilarly coupled into ports B and A, with the coupling between D and Bresulting in a loss of 3 dB with a 0 degree phase shift, and thecoupling between ports D and A resulting in a loss of 3 dB with a −90degree phase shift.

[0023] A band pass filter 20 protects the portion of the circuitoperating in the first frequency band, for example, the SatCom systemfrequency band, from RF energy in the second frequency band, forexample, the GPS frequency band, as well as other signal noise. Bandpass filter 20 is coupled between port B of coupler 18 and transceiver12. Band pass filter 20 is designed to pass RF energy which is withinthe first frequency band, for example, the SatCom system frequency band,and reflect energy which is outside that band.

[0024] A pair of band pass filters 22, 24 protect the portion of thecircuit operating in the second frequency band, for example, the GPSfrequency band, from RF energy in the first frequency band, for example,the SatCom system frequency band, as well as other signal noise. Bandpass filters 22, 24 are each designed to pass RF energy which is withinthe second, or GPS, frequency band, while reflecting RF energy which isoutside the second band. Filters 22, 24 couple respective ports C and Dof coupler 18 to ports A and B of another four-way directional coupler26 having essentially the same characteristics as coupler 18. Filter 22couples port C of first four-way directional coupler 18 to port A ofcoupler 26, and filter 24 couples port D of first coupler 18 to port Bof second coupler 26. RF energy introduced at respective ports A and Bof coupler 26 is recombined at port D of coupler 26. Port D of secondcoupler 26 is coupled into receiver 14 through a band pass filter 28, alow noise amplifier 30, and another band pass filter 32. Both band passfilters 28 and 32 are designed to reflect RF energy which is outside thesecond, or GPS, frequency band, and pass RF energy which is within thesecond band. Port C of second coupler 26 is grounded, preferably using a50 ohm resistor.

[0025] The diplexing/protection provided by isolation circuit 10 of thepresent invention is effected as shown in the FIGURE. Duringtransmission, an RF signal introduced from transceiver 12, such as aSatCom transceiver, is passed through band-pass filter 20, which cleansthe transmission signal in the first frequency band of harmonics in thesecond, or GPS, frequency band and spurious emissions. The cleaned RFsignal is introduced into coupler 18 at port B. Coupler 18 splits the RFenergy equally into two signals coupled into ports C and D. At ports Cand D of coupler 18, the two cleaned RF signals are coupled intorespective band pass filters 22 and 24. The frequency of the RF energyin the two cleaned RF signals is outside the pass range of both filters22 and 24, so each filter 22 and 24 reflects the same received RF energyback into coupler 18. The coupling and phasing on the reflected RFenergy is as described above, such that all of the RF energy in thecleaned RF signal is passed to port A of coupler 18 and output toantenna 16; none of the RF energy is reflected back to port B and thesignal generator, i.e., transceiver 12, and none of the RF energy ispassed through filters 22 or 24 to the portion of the circuit operatingin the second frequency band, for example, a GPS portion of the circuit.

[0026] An RF signal in the first frequency band, for example, the SatComfrequency band, received at antenna 16 follows a similar path totransceiver 12. An RF signal in the first frequency band received onantenna 16 is passed into coupler 18 at port A, and is coupled throughports C and D into respective band pass filters 22 and 24, as describedabove. The RF energy in the received first frequency band signal, or theSatCom frequency band signal, is outside the pass band of both filters22 and 24; therefore, filters 22 and 24 reflect the energy back intocoupler 18. As described above, the coupling and phasing on thereflected RF energy introduced back into coupler 18 at respective portsC and D passes all of the RF energy through band pass filter 20 to portB of coupler 18 and output, for example, to receiver 12 for processing.None of the RF energy is passed through filters 22 or 24 to the portionof the circuit operating in the second frequency band, for example, aGPS portion of the circuit, and none of the received RF energy isreflected back to port A and antenna 16.

[0027] An RF signal in the second frequency band, for example, the GPSfrequency band, received at antenna 16 is passed into first coupler 18at port A, and is coupled through ports C and D of coupler 18 intorespective band pass filters 22 and 24, as described above. The RFenergy of the received second frequency band signal, or GPS signal, iswithin the pass band of filters 22 and 24 and is passed through torespective ports A and B of second coupler 26. Second coupler 26recombines the RF energy introduced at respective ports A and B at portD, where the RF energy is passed into another band pass filter 28. TheRF energy is within the pass band of filter 28 and is passed to lownoise amplifier 30 for amplification. The amplified RF energy is passedby band pass filter 32, which is again designed to pass RF energy whichis within the second, or GPS, frequency band, as is band pass filter 28,as described above. The resulting second frequency band signal, or GPSsignal, is passed through filter 32 and output, for example, to receiver14 for processing.

[0028] While the preferred embodiment of the invention has beenillustrated and described, it will be appreciated that various changescan be made therein without departing from the spirit and scope of theinvention. For example, while the invention is illustrated using afrequency matched twin-line for coupling the RF energy, low-costthree-pole ceramic filters with a pass band centered at 1.575 GHz tofilter the RF energy, and a conventional GPS low noise amplifier withthe entire system laid out on a single RF substrate to provide impedancematching of the lines between components, the invention is not intendedto be limited to the disclosed embodiments. In one example, aconventional 3 dB branch-line coupler is tuned to the desired frequencyhas similar characteristics to four-way directional coupler 18 describedabove, except that it operates in a narrower frequency band and ispreferred for a narrow band application. Various other alternativeembodiments will be obvious to those of ordinary skill in the art andsuch alternative embodiments are intended to be within the scope of thepresent invention.

[0029] The embodiments of the invention in which an exclusive propertyof privilege is claimed are defined as follows:

1. A method for using a single antenna for simultaneously receivingradio frequency signals in a first radio frequency band and radiosignals in a second radio frequency band different from the first radiofrequency band, the method comprising: dividing the energy in a receivedradio frequency signal into two received signals; reflecting the energyin said two received signals in frequency bands other than the secondradio frequency band; and combining said energy in said two reflectedsignals into a first radio frequency output signal.
 2. The methodrecited in claim 1, wherein said received signal is a radio frequencysignal in the first radio frequency band; and further comprisingcleaning said received signal before said dividing the energy into tworeceived signals.
 3. The method recited in claim 2, further comprisingoutputting said first radio frequency output signal to an antenna. 4.The method recited in claim 3, wherein said received radio frequencysignal is a satellite communications system signal.
 5. The methodrecited in claim 4, further comprising, generating said satellitecommunications system signal.
 6. The method recited in claim 1, whereinsaid received signal is a radio frequency signal in he second radiofrequency band; and further comprising combining the energy in said tworeceived signals in the second frequency band into a second radiofrequency output signal.
 7. The method recited in claim 6, furthercomprising amplifying the combined energy.
 8. The method recited inclaim 7, wherein said received radio frequency signal is a GlobalPositioning System signal.
 9. The method recited in claim 8, furthercomprising outputting said combined and amplified second radio frequencyoutput signal to a Global Positioning System signal receiver.
 10. Amethod for using a single antenna for simultaneously receiving firstradio frequency signals in a first radio frequency band and second radiosignals in a second radio frequency band different from the first radiofrequency band, the method comprising: introducing a first receivedradio frequency signal in the first radio frequency band into a firstport of a coupler; dividing the energy in the first received signal intotwo first received signals; introducing each of said two first receivedsignals into second and third ports of said coupler; reflecting theenergy in said two first received signals in frequency bands other thanthe second radio frequency band; combining said energy in said tworeflected first received signals into a first radio frequency bandoutput signal; introducing a second received radio frequency signal in asecond radio frequency band different from the first radio frequencyband into said first port of said coupler; dividing the energy in thesecond received signal into two second received signals; introducingeach of said two second received signals into second and third ports ofsaid coupler; and combining the energy in said two second receivedsignals in the second radio frequency band into a second radio frequencyband output signal.
 11. The method recited in claim 10, furthercomprising: introducing a transmission radio frequency signal in thefirst radio frequency band into a fourth port of a first coupler;dividing the energy in said transmission signal into two transmissionsignals, introducing each of said two transmission signals into saidsecond and third ports of said first coupler; reflecting the energy insaid two transmission signals in frequency bands other than the secondradio frequency; and combining said energy in said two reflectedtransmission signals into a first radio frequency band transmissionsignal.
 12. The method recited in claim 11, further comprising cleaningsaid transmission radio frequency signal of radio frequency energy otherthan radio frequency energy in the first radio frequency band beforesaid introducing.
 13. The method recited in claim 11, wherein said firstradio frequency band transmission signal is a satellite communicationssystem transmission signal.
 14. The method recited in claim 13, furthercomprising outputting said first radio frequency band transmissionsignal to an antenna.
 15. The method recited in claim 14, furthercomprising generating said transmission radio frequency signal.
 16. Themethod recited in claim 10, wherein said second received radio frequencysignal is a Global Positioning System signal.
 17. The method recited inclaim 16, further comprising outputting said second radio frequency bandoutput signal to a Global Positioning System signal receiver.
 18. Themethod recited in claim 10, wherein said coupling the energy in said twosecond received signals in the second radio frequency band into areceiver further comprises passing the energy in said two secondreceived signals in the second radio frequency band introduced into saidsecond and third ports of said coupler.
 19. The method recited in claim18, wherein said combining the energy in said two second receivedsignals in the second radio frequency band into said second radiofrequency band further comprises amplifying the combined energy.
 20. Afrequency isolation circuit for simultaneously receiving radio frequencysignals in different first and second radio frequency bands, the circuitcomprising: a directional coupler configured to direct a signal input atfirst port into one or more second ports; and one or more filterscoupled to receive the output of each said second port, said one or morefilter configured to reflect radio frequency energy in radio frequencybands other than the second radio frequency band; and said directionalcoupler further configured to direct said reflected radio frequencyenergy into another port of said directional coupler.
 21. The frequencyisolation circuit recited in claim 20, wherein said directional coupleris further configured to direct a signal input at said other port intosaid one or more second ports; and to direct said reflected radiofrequency energy from said signal input at said other port into saidfirst port of said directional coupler.
 22. The frequency isolationcircuit recited in claim 21, further comprising a filter coupled to saidother port, said filter passing radio frequency energy in the firstradio frequency band.
 23. The frequency isolation circuit recited inclaim 22, further comprising an antenna coupled to said first port ofsaid directional coupler.
 24. The frequency isolation circuit recited inclaim 23, further comprising a transmitter coupled to said other portthrough said filter, said transmitter generating said input signal. 25.The frequency isolation circuit recited in claim 20, wherein said one ormore filters coupled to receive the output of each said second port arefurther configured to pass radio frequency energy in the second radiofrequency band.
 26. The frequency isolation circuit recited in claim 25,further comprising a second coupler coupled to receive the output ofeach said second port, said coupler combining said radio frequencyenergy in the second radio frequency band into a combined output signal.27. The frequency isolation circuit recited in claim 26, furthercomprising an amplifier coupled to receive said combined output signalof said second coupler.
 28. The frequency isolation circuit recited inclaim 27, wherein said amplifier is a low noise amplifier.
 29. Thefrequency isolation circuit recited in claim 27, further comprising areceiver coupled to receive the amplified output of said amplifier. 30.A frequency isolation circuit for simultaneously receiving radiofrequency signals in different first and second radio frequency bands,the circuit comprising: a four-way directional coupler having a firstport coupled to each of a second port and a third port; first and secondband pass filters coupled to respective ones of said second and thirdports, each said first and second band pass filter reflecting radiofrequency energy in the first radio frequency band back into saiddirectional coupler and passing radio frequency energy in the secondradio frequency band; said directional coupler combining said reflectedradio frequency energy into an output signal at a fourth port.
 31. Thefrequency isolation circuit recited in claim 30, wherein said second andthird ports are each coupled to a fourth port of said directionalcoupler.
 32. The frequency isolation circuit recited in claim 31,wherein: said coupling between said first port and said second portresults in a loss of 3 dB with a 0 degree phase shift; said couplingbetween said first port and said third port results in a loss of 3 dBwith a 90 degree phase shift, said coupling between said fourth port andsaid second port resulting in a loss of 3 dB with a −90 degree phaseshift; said coupling between said fourth port and said third portresulting in a loss of 3 dB with a 0 degree phase shift; said couplingbetween said second port and said first port resulting in a loss of 3 dBwith a 0 degree phase shift; said coupling between said second port andsaid fourth port resulting in a loss of 3 dB with a −90 degree phaseshift; said coupling between said third port and said fourth portresulting in a loss of 3 dB with a 0 degree phase shift; and saidcoupling between said third port and said first port resulting in a lossof 3 dB with a −90 degree phase shift.
 33. The frequency isolationcircuit recited in claim 32, further comprising a third band pass filtercoupled to said fourth port of said directional coupler, said third bandpass filter passing radio frequency energy in the first radio frequencyband.
 34. The frequency isolation circuit recited in claim 33, furthercomprising a transmitter generating a radio frequency signal in thefirst radio frequency band.
 35. The frequency isolation circuit recitedin claim 34, wherein said transmitter is a transceiver generating aradio frequency signal in the first radio frequency band and receivingsaid output signal at said fourth port of said directional coupler. 36.The frequency isolation circuit recited in claim 32, further comprisinga second coupler having an output port and input ports coupled torespective ones of said first and second band pass filters, said secondcoupler combining said passing radio frequency energy in the secondradio frequency band passed by said first and second band pass filtersinto an output signal at said output port.
 37. The frequency isolationcircuit recited in claim 36, further comprising an amplifier coupled tosaid output port of said second coupler.
 38. The frequency isolationcircuit recited in claim 37, further comprising a single radio frequencysubstrate having said directional coupler, said first, second and thirdfilters, and said amplifier laid thereon.
 39. The frequency isolationcircuit recited in claim 37, wherein said amplifier is a low noiseamplifier.
 40. The frequency isolation circuit recited in claim 36,further comprising a radio frequency receiver coupled to said outputport of said second coupler for receiving said output signal.
 41. Thefrequency isolation circuit recited in claim 40, wherein said radiofrequency receiver coupled to said output port of said second couplerfor receiving said output signal is a Global Positioning Systemreceiver.